Use of pesticides for agriculture and other purposes has increased greatly in the last 50 years, posing an increasing risk both to occupational groups manufacturing or using pesticides and to the general population. Clinical manifestations of pesticide neurotoxicity are well known, but there is growing concern that pesticides may also produce subclinical neurologic deficits, particularly as a delayed consequence of a poisoning episode or in situations of chronic, low-level exposure. Such effects might degrade an individual's quality of life or be a prelude to clinically detectable dysfunction or frank neurologic disease. Although case reports of pesticide poisoning are common, and neurologic deficits may be apparent years after a poisoning episode, few studies have investigated the neurologic effects of chronic low-dose exposure. Among occupational groups potentially exposed to pesticides, farmworkers are unique. They work in intimate contact with treated crops on a daily basis, often without protective clothing or adequate sanitary facilities. Laws regulating their exposure may not be enforced. Farmworkers often live in or near areas treated with pesticides. Poor access to medical care may compound the effects of chronic exposure. The health of this highly exposed group has never been adequately studied, partly because of the logistical problems involved. Farmworkers often move frequently, they may not speak English, and they may have little formal education. These factors make it difficult to conduct an epidemiologic study with a defined population and good response rates. To address the scientific issue of pesticide neurotoxicity and to investigate health effects of pesticides in this underserved group, we studied the relationship of pesticide exposure to neurologic dysfunction in a group of farmworkers in central Florida. In order to overcome the logistical problems often encountered in studies of migrant farmworkers, we identified a stable population and enlisted the collaboration of a local farmworker organization. Our study represents an advance over previous studies in several ways: we recruited exposed farmworkers and unexposed controls from the same population; we used a broad range of neurologic tests to assess cognitive, motor, and sensory function; we included a detailed evaluation of chronic exposure; and we archived DNA for genetic analyses. Working with a defined population in central Florida, we studied 283 farmworkers who had worked on one of three crops -- ornamental ferns, nursery plants, or citrus fruit -- and compared them to 51 unexposed controls. Information on farmwork history was collected using a structured questionnaire. Neurobehavioral performance was evaluated using a battery of seven tests adapted for use in Latino populations; some tests were implemented on a computer. Data were analyzed using logistic or linear regression, adjusted for potential confounders including age, sex, height, weight, education, acculturation, and alcohol use. The recruiting process was generally successful. The overall response rate was 72%. We compared eligible individuals who participated to those who did not. The two groups did not differ by age or sex. However, individuals who had done farmwork for 6 to 15 years were more likely to enroll than those who had done farmwork for either less or more time. Workers in one of the defined types of agriculture (nursery, citrus, or ferns) were more likely to enroll than either unexposed controls or those who had worked in various types of agriculture for 15 or more years. We have begun analyses to compare exposed farmworkers to unexposed controls. Fern work was associated with worse performance for digit span, symbol digit, tapping, Santa Ana, and postural sway; nursery and citrus work were associated with worse performance for tapping and postural sway. No changes in vibrotactile sensitivity or grip strength were related to farmwork. Deficits associated with farm work, when present, were comparable in size to those associated with less education or increased alcohol use. Dose-response for months of work was observed for fern work for tapping and Santa Ana; for nursery work for postural sway; and for citrus for tapping. We conclude that work with ferns, nursery plants, and citrus fruit may be associated with deficits in neurobehavioral performance reflecting cognitive, psychomotor, and integrated sensory-motor function.